A LOW-NOISE TTL-COMPATIBLE CMOS OFF-CHIP DRIVER CIRCUIT

Low-noise TTL-compatible off-chip driver (OCD) circuits are very impor tant, especially for low-power electronics, but scaled-down CMOS techn ology requires a lower operating voltage of 3.3 V, while most applicat ions require 5 V. The dual power-supply requirement makes the design o f OCD challenging, first because pull-up devices, especially p-MOS dev ices, must be able to handle an off-chip voltage of 5.6 V, which is hi gher than an on-chip V-DD of 2.8 V, and second because pull-down devic es should be able to discharge a capacitive load of 5.6 V while operat ing at a minimum on-chip V-DD of 2.8 V. This extreme difference in ope rating voltage makes the circuits susceptible to ringing and performan ce degradation due to hot-electron effects. In this paper, we describe a low-noise OCD which has been successfully used in IBM second-genera tion 4Mb low-power DRAM (LPDRAM) and in other products. For pull-ups, two stacked p-MOS devices with floating n-wells are used, but they are operated in different modes depending on the supply voltage. The pull -down devices are basically composed of two stages, one of which is in the diode configuration with its gate and drain shorted together duri ng the pull-down. Detailed circuit designs to achieve low noise while meeting the performance requirements are described.